1. Field of the Invention
The present invention relates to interferometers used in Fourier Transform spectrometers and driving devices for moving the mirrors therein.
2. Description of Related Art
A multibeam interferometer for use in a Fourier Transform spectrometer such as shown in FIG. 16 has been known.
Referring now to FIG. 16, a light source 51 emits a light beam 68 to a beam splitter 52 which splits the beam, directing a first beam to a fixed flat mirror 53, and a second beam to a movable flat mirror 54. Movable flat mirror 54 is moved back and forth in direction 69, in parallel to the optical axis of the transmitted beam 68.
The fixed flat mirror 53 and the movable flat mirror 54 are arranged so that the angle of incidence for each beam thereupon is 0 degrees, in order to make the optical path of the incident beam almost identical with the optical path of the emitted beam, thereby making the respective beams reflected by the fixed flat mirror 53 and the movable flat mirror 54 equally incident upon the beam splitter 52 again.
With this interferometer, the movable flat mirror 54 is moved linearly 69 in parallel to the optical axis of the incident beam in order to change the optical path length of the beam transmitted from the beam splitter 52. This produces an optical path difference between the incident beam and the reflected beam so that these beams do not arrive at the beam splitter 52 at the same time, and thus cause an interference pattern to occur. The interference pattern is detected at a target 55.
At the same time, it is necessary to provide an angle of inclination for the movable flat mirror 54 that is reduced to 1 degree or less, in order to make the optical axis of the beam incident upon the movable flat mirror 54 almost parallel to the optical axis of the beam emitted from the movable flat mirror 54. A highly accurate air bearing or a parallelogramic linking mechanism has been used as a driving device (not shown) for linearly moving the movable flat mirror 54. The driving device disclosed in Japanese Patent Application Laid-Open No. Sho 63-501174 is one example of such a prior art device.
The driving device disclosed in this publication utilizes a pair of parallel links pivotally mounted on a fixed member at their end portions. A movable flat mirror is mounted on a swinging member pivotally mounted on the other ends of these parallel links. This swinging member is reciprocated by a linear motor to move the movable flat mirror.
Another prior art interferometer is shown in FIG. 17. Referring to FIG. 17, a movable mirror 54A is shown receiving a transmitted beam 70 from a beam splitter 52. The movable mirror is composed of a cube corner mirror comprising three pieces of flat mirror positioned vertically adjacent to each other and moving linearly 71 in parallel to an optical axis of the incident transmitted beam 70. A fixed flat mirror 56 returns the beam incident thereupon from the movable mirror 54a to the movable mirror 54a. Like parts in FIGS. 16 and 17 are designated by the same reference numerals.
In this interferometer, as in the one shown in FIG. 16, the movable mirror 54a is moved linearly to produce a difference in optical path length between the reflected beam and the beam transmitted from the beam splitter 52, so that an interference pattern occurs at beam splitter 52.
The prior art interferometer shown in FIG. 18 is known as an interferometer that does not require a support and guide mechanism for linearly moving a movable flat mirror as required for the structure of FIGS. 16 and 17.
Referring to FIG. 18, a light source 61 directs a beam at a beam splitter 64 mounted on a swinging plate 62. Plate 62 also has a pair of movable flat mirrors 63a, 63b fixedly mounted thereon in parallel with opposing reflecting surfaces, so that the reflecting surfaces are two opposite sides of a parallelogram with the beam splitter 64 fixedly located in parallel with the movable flat mirrors 63a, 63b between the two reflecting surfaces of the flat mirrors 63a, 63b.
The swinging plate 62 is rotatably supported by a supporting shaft (not shown). It is swung, as shown by an arrow 72. The axis of the supporting shaft is arranged in parallel to the surface of the movable flat mirrors 63a, 63b. A fixed flat mirror 65 is located for reflecting a beam, which is incident thereupon from beam splitter 64 through the movable flat mirror 63a, over the same optical path to make the reflected beam incident upon the movable flat mirror 63a again. A fixed flat mirror 66 reflects a transmitted beam, which is incident thereupon from the beam splitter 64, through the same optical path. Reference numeral 67 designates a detector.
With this interferometer, a reflected beam from the beam splitter 64 is reflected by the movable flat mirror 63a and the fixed flat mirror 65 to be incident upon the beam splitter 64 again. A transmitted beam from the beam splitter 64 is reflected by the flat mirror 66 to be incident upon the beam splitter 64 again, but the movable flat mirrors 63a, 63b and the beam splitter 64 are rotatably swung at the same time.
Accordingly, the optical path length from the light source 62 to the flat mirror 65 of the reflected beam is changed to produce a difference between the optical path length of the reflected beam and an optical path length of the transmitted beam having the constant optical path length, so that an interference occurs when they are incident upon the beam splitter 64 again.
In the conventional interferometer of FIG. 16, it is essential that the angle of inclination of the movable mirror 54 be reduced to about 1 degree. Consequently, a highly accurate air bearing or a parallelogram link mechanism has been used as the driving device for the movable flat mirror 54.
If an air bearing is used, it is necessary to provide an air supply. This complicates the operation and makes the apparatus expensive. In addition, it is necessary for the parallelogram link mechanism to move the lengths of the respective opposite sides exactly equal to each other. This requires a high dimensional accuracy for the respective links, causing difficulty in manufacturing such a high accuracy mechanism and increasing the cost of the interferometer. In particular, in prior art mechanisms having a movable flat mirror 54 driven by means of a linear motor, as disclosed in said Japanese Patent Application Laid-Open No. Sho 63-501174, electric power consumption has been high, and the space required for the installation of the driving mechanism is larger than desirable.
Moreover, since every driving device for this application requires a high accuracy, a slight deformation of the respective parts resulting from a temperature change and the like influences the support of the movable flat mirror 54 to change the angle of inclination of flat mirror 54. This increases the shift between a beam incident upon the flat mirror 54 and a beam emitted from the flat mirror 54.
Since the reflected beam and the transmitted beam from the beam splitter 52 travel along different paths, the reflected beam and the transmitted beam pass through spaces that vary in the turbulent condition of air. Accordingly, a difference can be produced between a wave surface of the reflected beam and that of the transmitted beam due to the difference in turbulence of the air. As a result, when the reflected beam and the transmitted beam are rejoined at the beam splitter 52, there is also a greater possibility that the turbulence variation will appear as a noise on an interferogram produced by a Fourier spectrometer.
In the prior art example shown in FIG. 17, since a cube corner mirror 54a is used as the movable mirror, the angle formed between the incident beam and the emitted beam by the inclination of the cube corner mirror produced when it is linearly moved is reduced. Thus, the accuracy required of a driving device for driving the movable mirror is decreased in comparison with that required for the interferometer shown in FIG. 16.
However, a remarkably high accuracy is required of the cube corner mirror 54a used for the movable mirror. That is to say, it is necessary for three pieces of flat mirror to be adjacent to each other while maintaining a completely vertical relationship with a permissible value of a difference in angle in vertical relationship being 1 degree or less. It is difficult to obtain highly accurate cube corner mirrors. A highly accurate cube corner mirror is expense, thereby remarkably raising the cost of an interferometer.
Although a roof-shaped mirror comprising two pieces of flat mirror vertically adjacent to each other acts in the same manner as the cube corner mirror, it also must be manufactured with a high degree of accuracy.
Since the reflected beam and the transmitted beam from the beam splitter 52 passes through difference space, noise resulting from a difference in turbulence of air in the respective spaces occurs in the same manner as in the conventional interferometer shown in FIG. 16.
In the interferometer of FIG. 18, movable flat mirrors 63a, 63b and beam splitter 64 are rotatably swung by means of swinging plate 62. Improving the accuracy of a rotating and supporting mechanism, the shaft, one which the swinging plate 62 moves, is easier than improving the accuracy of the driving device in the interferometers of FIGS. 16 and 17. Thus the cost of the supporting mechanism for the driving device can be reduced. Also, the above-noted problems resulting from use of the linear movement driving devices can almost be solved.
However, in the interferometer of FIG. 18, the movable flat mirrors 63a, 63b and beam splitter 64 are rotatably swung by means of said swinging plate 62 to only change the optical path length of a reflected beam. A problem occurs in this embodiment in that this makes it difficult to increase the difference beam the optical path length of the reflected beam and the transmitted beam. Since the reflected beam and the transmitted beam pass through different space, the problem of noise resulting from different air turbulence in the respective spaces is still present in the same manner as for the conventional interferometers of FIGS. 16 and 17.
Moreover, in the interferometer of FIG. 18, the swinging plate 62 is rotatably swung by means of a servomotor. This causes a high consumption of electric power and requires a larger than desirable space for the installation of the driving device.
The present invention solves the above-described problems in an interferometer, in which movable mirrors are rotatably swung, as shown in FIG. 18.